In order to achieve a highlight color display, color filters are often used. In one of the obvious options, each pixel has two sub-pixels and the two sub-pixels are based on two display cells capable of displaying black and white color states and only one of the sub-pixels has a color filter overlaid on top of a display cell. When a color state (e.g., red, green or blue) is desired, the sub-pixel with the color filter is turned on and the sub-pixel without a color filter is turned to the white or black state. When the black state is desired, both sub-pixels are turned to the black state. When the white state is desired, the sub-pixel without a color filter is turned white and the sub-pixel with a color filter is turned black. It is fairly obvious that the disadvantage of such a technique is that the maximum reflectivity of the white state is only 50% since the sub-pixel with the color filter must be turned black for that state. Conversely the color state is of a dark shade (with the black turned on) or a light shade (with the white turned on).
An alternative technique utilizing a dual switching mode is proposed in U.S. Pat. No. 7,046,228. With the dual switching mode, an extra set of electrodes are added to each microcup so that the microcup can be switched into three states; white (with the particles at top), colored (with the particles at bottom), and black (with the particles moved to the sides). A black background layer is used so when the particles are moved sideways, the black state shows through the microcup. The advantage of this is that high quality white and black states can be achieved. In addition, each microcup defines a pixel and no sub-pixels are required. However, a special electrode structure is needed for this dual mode switching.